An aluminum heat exchanger used in automobile air conditioner, from the viewpoint of a heat exchange efficiency improvement, usually has a plurality of fins arranged at narrow intervals in order to make the surface area thereof as large as possible, as well as tubes for coolant supply arranged tangled in these fins. With the heat exchanger of such a complicated structure, the moisture in the atmosphere during air-conditioner operation adheres as condensed water on the surfaces of the fins and tubes (hereinafter referred to as “fins, etc.”). However, in the case of the wettability of the surfaces of the fins, etc. is inferior, the ventilation resistance increases by, for example, the adhered condensed water becoming substantially semispherical droplets and being present in a bridged form between the fins, a result of which there is a problem in that the smooth flow of evacuated air is inhibited, and the heat exchange efficiency declines. In order to suppress such a phenomenon, normally, hydrophilization treatment has been conducted on the surfaces of the fins, etc.
In addition, the aluminum or alloys thereof constituting the fins, etc. are essentially materials excelling in rust-prevention. However, when condensed water stagnates for a long time on the surfaces of fins, etc., oxygen concentration cells are locally formed and the corrosion reactions advance, and if contaminant components in the atmosphere further adhere thereto, the corrosion reactions will be promoted. The products generated from the corrosion reactions, for example, white rust, deposits on the surface of fins, etc., thereby the heat exchange characteristic being inhibited, and further, there are problems such that the white rust is discharged to the atmosphere by the blower fan.
For this reason, a variety of technologies for suppressing the generation of white rust and improving corrosion resistance have been proposed. For example, as a chemical conversion treatment agent to impart favorable corrosion resistance to surfaces of aluminum or alloy materials thereof, chemical conversion treatment agents have been disclosed that contain titanium-complex fluoride ions, pentavalent vanadium compound ions and zirconium-complex fluoride ions (refer to Patent Document 1).
In addition, as a chemical conversion treatment agent that imparts favorable corrosion resistance to the surfaces of an aluminum heat exchanger, chemical conversion treatment agents have been disclosed that contain decavanadate ions corresponding to the pentavalent vanadium compound ions and zirconium-complex fluoride ions (refer to Patent Document 2).
Here, the aluminum heat exchanger used in automobile air conditioner is manufactured by arranging and assembling the plurality of fins, etc. as mentioned above, and then joining these. Upon joining, a strong, dense oxide film is formed on the surface of aluminum; therefore, joining by way of a brazing method that is not a mechanical joining method is not easy, and thus schemes such as brazing in vacuum have been necessary.
In recent years, as a means to effectively remove the oxide film on the surface to address this, the flux brazing method using a halogen-based flux has been developed, and thereamong, the Nocolok brazing process of flux brazing in nitrogen gas (hereinafter referred to as “NB process”) is widely used from the viewpoint of the control of brazing is simple and the processing cost is inexpensive. With this NB process, after the plurality of fins, etc. are arranged and assembled, the fins, etc. are brazed in nitrogen gas using a flux such as KAlF4 and K2AlF5.
However, in the aluminum heat exchanger manufactured by the NB process (hereinafter referred to as “NB heat exchanger”), flux inevitably remains on the surfaces of the fins, etc. Given this, the surface state of the fins, etc. (electrical potential state, etc.) will be non-uniform, a result of which a uniform chemical conversion film and hydrophilized film cannot be obtained by subsequent treatments, and thus there has been a problem in that favorable corrosion resistance and hydrophilicity are not obtained.
Therefore, as a surface treatment method for a NB heat exchanger that imparts favorable odor resistance, which is an important characteristic as an automobile air-conditioning application, in addition to favorable corrosion resistance and hydrophilicity, technology has been disclosed that after the NB heat exchanger is subjected to a chemical conversion treatment by dipping in a chemical conversion treatment agent containing at least one among zirconium-complex fluoride ions and titanium-complex fluoride ions, then the NB heat exchanger is subjected to a hydrophilization treatment by dipping in a hydrophilization treatment agent containing polyvinyl alcohol, polyoxyalkylnene-modified polyvinyl alcohol, inorganic cross-linker, guanidine compound, etc. (refer to Patent Document 3).
In addition, as a surface treatment method that can maintain the hydrophilicity, high corrosion resistance, antimicrobial property and odor resistance on the surface of aluminum or an aluminum alloy material over a long time period, a technology has been disclosed that sequentially passes an aluminum or aluminum alloy substrate surface through a surface adjustment step to establish a state suited to the formation of a chemical conversion film, a water washing step, a step of forming a first protective layer consisting of the chemical conversion film on the surface of the aluminum or aluminum alloy substrate, a water washing step, a step of coating a second protective layer that is an organic film on the first protective layer, and a drying step (refer to Patent Document 4). With this technology, the first protective layer is formed by a chemical conversion treatment liquid containing vanadium and at least one type of metal selected from titanium, zirconium and halfnium, the second protective layer is formed with a composition containing (1) a chitosan derivative and solubilizer, (2) a modified-polyvinyl alcohol made by a hydrophilic polymer graft polymerizing at a side chain of polyvinyl alcohol, and (3) a water-soluble cross-linker.
In addition, as technology for imparting superior corrosion resistance to aluminum-based metal materials, etc., technology has been disclosed that is related to surface treatment agents with essential components of a resin compound having a specific structure, a vanadium compound, and a specific metal compound (refer to Patent Document 5). With this technology, it is said that, by containing ascorbic acid, etc. as the water-soluble organic compound having at least one functional group selected from the group consisting of a hydroxyl group, carbonyl group, carboxyl group, phosphate group, phosphonate group, primary to tertiary amino group and amide group, for example, not only is the vanadium compound reduced, but also the stability of the vanadium compound is remarkably improved, and a superior corrosion resistance imparting effect can be maintained over a long time period. In addition, it is said that a uniform film can be formed, and the level of corrosion resistance can be improved.
[Patent Document 1] Japanese Unexamined Patent Application, Publication No. 2010-261058
[Patent Document 2] Japanese Unexamined Patent Application (Translation of PCT Application), Publication No. 2004-510882
[Patent Document 3] Japanese Unexamined Patent Application, Publication No. 2006-69197
[Patent Document 4] Japanese Unexamined Patent Application, Publication No. 2011-161876
[Patent Document 5] Japanese Unexamined Patent Application, Publication No. 2001-181860